Letterman, to his credit, went on to ask Obama how much the national debt actually is. Obama evidently knew that if he said $16 trillion his audience would be horrified, so, incredibly, he pretended not to know! You have to see it to believe it:

Click over to Power Line for the clip as it went out over the airwaves. However, we obtained a video purporting to show the above moment from another angle. And although we can't fully vouch for the provenance of this hidden camera footage, we're showing it anyway, since it may indicate unique and heretofore unnoticed nuances into Mr. Obama's answer: